cmdutils: Use a configure check for enabling CommandLineToArgvW

cmdutils: Use a configure check for enabling CommandLineToArgvW

This simplifies the condition to avoid hardcoding the systems
where the function exists. This also simplifies support for
newer Windows API subsets where this function doesn't exist,
such as Windows Phone 8 and the "metro" API subset of Windows 8.

Signed-off-by: Martin Storsjö <martin@martin.st>